The present invention relates to a packaging tray made of semi-rigid material, for example corrugated cardboard, the inner walls of which are intended to be lined, by thermoforming, with a film of thermoplastic material. Such trays are currently used for packaging moist foodstuffs in a modified gaseous atmosphere in order to preserve them.
An example of this type of tray is known from U.S. Pat. No. 4,722,474. This tray is of the type constructed from a blank, comprising a substantially rectangular bottom, on the sides of which there are articulated two longitudinal walls provided with turn-in panels intended to be folded down in order to double the longitudinal walls, and two transverse walls provided with entablatures intended to extend parallel to the bottom and toward the inside of the assembled tray. Once assembled, the tray is made impermeable by a protective film which is applied without discontinuity on its internal faces. In order to make such a lining, the tray is placed in the assembled condition inside a mold, then a strip of film made of thermoplastic material is arranged on top of the opening of the mold. A cover is pressed over the opening of the mold, the edges of the film strip then being applied against the top faces of the entablatures, the film is subsequently heated to bring it to its softening point, and is then blown or sucked toward the inside of the tray by the application of pressure.
The edges of the film thus adhere to the top faces of the entablatures, whereas its central part expands until it is applied against the bottom faces of the entablatures and against the side walls and the bottom of the tray. The tray is subsequently sealed by means of a leaktight cover made of thermoplastic material.
However, it has been found that the lining film made of thermoplastic material is subjected to considerable stretching especially in the corners formed by the walls and the bottom, and this increases the taller the tray. In these regions, the film undergoes a great deal of thinning and may well tear during or after the thermoforming. In the event of the film being ruptured, the gas within the tray escapes and the foodstuffs are no longer under a conditioned atmosphere. Moreover, the external moisture or that which results from the sweating of the foods may reach the cardboard through the holes and tears in the film, rapidly reducing its resistance to stacking.